Examples of the constant velocity universal joints include a fixed type constant velocity universal joint, such as a Rzeppa type constant velocity universal joint (BJ) and an undercut free type constant velocity universal joint (UJ), and a plunging type constant velocity universal joint, such as a double offset type constant velocity universal joint (DOJ) and a cross groove type constant velocity universal joint (LJ).
The fixed type constant velocity universal joint of the Rzeppa type includes an outer joint member having an inner spherical surface equiangularly provided with a plurality of track grooves formed along an axial direction, an inner joint member having an outer spherical surface equiangularly provided with a plurality of track grooves formed along the axial direction in pairs with the track grooves of the outer joint member, a plurality of balls interposed between the track grooves of the outer joint member and the track grooves of the inner joint member so as to transmit torque, and a cage interposed between the inner spherical surface of the outer joint member and the outer spherical surface of the inner joint member so as to hold the balls.
The outer joint member and the inner joint member of such a constant velocity universal joint are manufactured by the method as follows . First, a columnar billet is formed by hot forging, warm forging, or cold forging into a schematic shape of the outer joint member or the inner joint member, and then subjected to a turning process into an arbitrary shape. After that, the columnar billet is subjected to heat treatment, and the outer spherical surface, the inner spherical surface, and the track grooves are subjected to a finishing process such as grinding and quenched-steel cutting.
The finishing process performed on the track grooves after forging, turning, and heat treatment as described above involves inconveniences such as increase in cost of equipment and tools required for the finishing process on the track grooves, time periods required for the finishing process, and a low material yield. As a countermeasure, there has been disclosed a method in which at least one of or both the track grooves of the outer joint member and the track grooves of the inner joint member are formed by cold-forging finishing (Patent Literature 1). When the track grooves are formed by cold-forging finishing as disclosed therein, the track grooves are formed only by cold-forging finishing. Thus, various machining processes such as a cutting process, a grinding process, and the like that have been conventionally performed after cold forging can be omitted. As a result, yields can be increased, and cost of the constant velocity universal joint can be reduced.